Process of treating metals to inhibit excessive grain growth



Patented Apr. 20, 1926.

UNITED STATES PATENT OFFICE.

GEORGE L. K ELLEY, OF PHILADELPHIA, PENNSYLVANIA, ASSIGNOR TO EDWARD G.

BUDD MANUFACTURING COMPANY, OF PHILADELPHIA, PENNSYLVANIA, A COR- PORATION OF PENNSYLVANIA.

PROCESS OF TREATING METALS TO INHIBIT EXCESSIVE GRAIN GROWTH.

No Drawing.

To all whom it may concern.

Be it known that I, GEORGE L. KELLEY, a citizen of the United States, and a resident of Philadelphia, county of Philadelphia, State of Pennsylvania, have made a certain new and useful Invention in Process of Treating Metals to Inhibit Excessive Grain Growth, of which the following is a specification.

This invention relates to the treatment of metals.

The object of the invention is to provide a process of treating metals, and more particularly a process for inhibiting the development of excessive grain growth.

The invention consists substantially in the mode of operation hereinafter more fully 5 tropic transformations occur in steel, Criti-.

I gerated grain growths.

pointed out and set forth in the appended claims.

The term critical temperature herein referred to is that boundary temperature below which exaggerated gram growth" will not occur. It is different from and independent of those definite temperatures at which allocal temperature for any given material is that temperature which, when present concurrently with critical strain, produces an exaggerated grain growth. And conversely, critical strain is that strain of which the critical temperature is afunction and the presence of which influences the exagowth of, ain. Strain is always produce in materia s b cold working processes. This strain on y becomes critical, however, when it is f such character that when present during the range of critical temperature an exaggerated rain growth is produced. For instance, hig carbon steels having neither critical strain nor critical temperature do not evidence exaggerated on the other hand, low caron steels, in which may be present-both critical strain and critical temperature, will evidence exaggerated grain growths when and onl when for a given critical strain the .steel is rought up to a certain temperature within the critical range.

When carbon steel, as Well as other metals, of the usual composition, is subjected to cold working it is ut into a condition the characteristic 0 which is that when sub- Application filed April 23, 1924. Serial No. 708,573.

sequently subjected to annealing temperatures, an abnormally large crystal or grain growth is caused therein. For example, in the case of carbon steel of the usual composition of carbon contentof between .05 and .15 percent, when subjected to a slight amount of cold working, such, for instance, as would correspond to a reduction in thickness thereof of from three to twelve percent, or more, it is said to be put in the condition of critical strain having the characteristic referred to when subjected for even short periods of time, say fifteen minutes, to temperatures between 1200 ,F. and, 1400 F that is, an abnormally large crystal or grain growth develops therein. Other metals, likewise, when similarly treated, are put in a corresponding condition of critical strain having the like characteristic.

Discussing further this formation of grain growth, abnormally large grains are known :to be caused by grain size contrast, strain gradients, temperature gradients, concentration gradients and obstruction gradients. Among the list of possible causes for such grain growth the one chiefly operative here and With which I am primarily concerned is the strain gradient. As above mentioned, the amount of strain is critical. are generally not uniform throughout a given piece. This gives the ideal strain gradient necessary to roduce the result. The greater the strain t e lower the temperature at which such strains are relieved. By using a temperature below that at which exaggerated grain growth occurs, it is possible to relieve those strains which are greatest in amount. This has the efi 'ect of reducing Such strains the previously existing strain gradient. If I gauge, though my invention is not to be limited in respect to the character of metal nor to the shape or form in which it is produced.

The development of an abnormally large crystal or grain growth in the annealing operation is undersirable and is to be avoided, So far as I am aware no effort has heretofore been directed towards the problem of inhibiting such abnormally large grain or crystal growths in metals, although the existence of this characteristic and its undesirability has long been recognized.

I have discovered that the excessive grain growth in metals is inhibited if the cold worked metal, before being subjected to a temperature, in annealing it, such as would develop the undesired excessive grain or crystal growth, is preliminarily heated to a degree below that at which such excessive grain growth develops, and is maintained at such preheating temperature for a suitable period of time. When this course is followed the subsequent application of an annealing temperature of such degree as to ordinarily cause the development of excessive crystal or grain growth will not cause such excessive growth in the metal.

The desired preheating may be effected in various ways in securing the desired re sults. If the metal, in the annealing 0peration, is heated at a sufficiently slow rate as to secure substantially the desired ,pre-heating effects before the annealing temperature is attained theexcessive growth inhibition is secured to an appreciable extent. 1n practice, however, and in attaining the best and most certain and reliable results, L prefer to first preheat the'metal to a predetermined degree of temperature below that at which the excessive grain growth develops, and to maintain the metal at that predetermined temperature for a pause or a period of sufficient duration, and then to increase the temperature to the required annealing point, or to the point at which such excessive grain growth would otherwise develop.

As one illustrative operation in carrying my invention into practical use, carbon steel having the carbon content, and which has been cold worked to reduce its section, as hereinbefore referred to, is first preheated to, say 1100 F., and is maintained at that temperature, say for a period of two hours. It then the temperature is increased to, say, 1200 F. to 1400 F. for annealing purposes,

the excessive crystal or grain growth will- The application of higher annealing tem-' peratures tends to desirably soften the resulting product.

The metals which may be treated by my process are those of the so-called ductile class, such as aluminum, zinc, nickel, German silver and the low carbon steels. Principally, however, I am interested in aluminum and the low carbon steels, which latter may or may not include among their constituents alloying elements such as nickel, chromium, vanadium and molybdenum.

It is to be understood that the illustrative operation above referred to is intended to be merely one example and not to impose a limitation upon the broadest scope of my invention as defined in the broad claims.

Having now set forth the objects, purpose and nature of my invention, and the manner of carrying the same into practical-use, what I claim as new and useful, and of my own invention, and desire to secure by Letters Patent is 1. The process which consists in heating metal to a degree of temperature below that at which excessive grain growth develops, and then increasing the temperature to a higher degree.

2. The process which consists in heating cold worked metal to and maintaining the same at .a temperature below that at which excessive clrystal or grain growth develops, and then increasing the temperature to a higher degree.

3. The process which consists in heating metal to and maintaining the same at a temperature below the annealing point, and

then increasing the heat to the complete annealing point.

4;. The process which consists in preliminarily heating cold worked metal to and maintaining the same at a temperature below that required for annealing and then raising the temperature to said annealing point.

5. The process which consists in cold working carbon steel, then preliminarily heating the same to a temperature below that necessary for annealing, and finally increasing the temperature to said annealing point.

' In testimony whereof I afiix my signature,

GEORGE L. KELLEY.

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